Aside from uncoated cutting tools of hard metals, cermets or ceramics, coated cutting inserts have developed into considerable significance in metal machining. It is known, for example, to provide shaped bodies which are comprised of at least the carbide of tungsten, titanium, niobium and tantalum, as well as metallic cobalt, as the binder, whereupon one or more surface layers can be applied by CVD, PVD or plasma CVD processes. These can be comprised of hard carbides, nitrides or carbonitrides of the elements titanium, zirconium or hafnium, or also of aluminum oxide.
It is especially known also to provide multilayer coatings of the layer sequence TiC, Ti(C,N), and TiN, optionally with an outer Al.sub.2 O.sub.3 layer. With such coated cutting tools, high machining rates and long working lives can be achieved when in turning, milling or boring of the workpiece a lubricating liquid is supplied for corresponding cooling. The cooling lubricant liquid uses mainly an aqueous emulsion whose main component, namely water to 95% effects the requisite cooling of the cutting edge. The emulsified or solubilized additives are necessary to supply the requisite lubrication during the machining process, for resisting corrosion and to protect the workpiece against chemical attack. It is a drawback of the mentioned additives that they are environmentally unfriendly. Indeed it has been found that toxic additives like chloroparaffins or benzotriazol should be replaced by less environmentally hazardous substances and simultaneously that the recycling or disposal of the cooling lubricant, are associated with not insignificant costs which may not be overcome as a consequence of the increased machining efficiency and the higher useful lives of the tools.
From an economical viewpoint it is advantageous to operate without a cooling lubricant in a dry mode and to be satisfied with a higher tool wear. This latter approach is largely practiced for example with short-chip workpieces like those of cast steel. With long-chip steels, the removed chip contacts the cutting tool over a large region and because of the increased contact area at the cutting wedge of the cutting tool, increased heat development arises from friction. A solution known in the art is to provide low-friction coatings of the cutting tool, for example of aluminum oxide, to minimize the cutting tool wear. This approach does not provide general satisfaction when the operation uses a cooling lubricant.
In EP 0 634 905 B1 it is proposed to provide workpieces with "soft" layers of a chalcogenide compound like molybdenum disulfide or molybdenum diselenide. Such layers can only be produced by means of a costly PVD process and increase, because of reduced abrasion resistance, the useful life of the cutting tool in the case of steel machining only marginally.